1. Field of the Invention
The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medicine images of a patient's body organs of interest. In particular, the present invention relates to a novel detector configuration for nuclear medical imaging systems that are capable of performing either positron emission tomography (PET) or planar and single photon emission computed tomography (SPECT).
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
Two basic types of imaging techniques are PET or “coincidence” imaging, and single photon imaging, also known as planar or SPECT imaging. PET imaging is fundamentally different from single photon imaging. In PET, events are detected from the decay or annihilation of a positron. When a positron is annihilated within a subject, two 511 KeV gamma rays are simultaneously produced which travel in approximately opposite (i.e., 180°) directions. Two scintillation detectors are positioned on opposite sides of the patient such that each detector will produce an electrical pulse in response to the interaction of the respective gamma rays with a scintillation crystal. In order to distinguish the detected positron annihilation events from background radiation or random events, the events must be coincident (i.e., both occur within a narrow time window) in each detector in order to be counted as “true” events. When a true event is detected, the line connecting the positions of the two points of detection is assumed to pass through the point of annihilation of the positron within the subject being imaged.
By contrast, single photon imaging, either planar or SPECT, relies on the use of a collimator placed in front of a scintillation crystal or solid state detector, to allow only gamma rays aligned with the holes of the collimator to pass through to the detector, thus inferring the line on which the gamma emission is assumed to have occurred. Both PET and single photon imaging techniques require gamma ray detectors that calculate and store both the position of the detected gamma ray and its energy.
Present day single photon imaging systems all use large area scintillation detectors (on the order of 2000 cm2). Such detectors are made either of sodium iodide crystals doped with thallium (NaI(Tl)), or cesium iodide (CsI). Scintillations within the NaI crystal caused by absorption of a gamma photon within the crystal, result in the emission of a number of light photons from the crystal. The scintillations are detected by an array of photomultiplier tubes (PMTs) in close optical coupling to the crystal surface. Energy information is obtained by summing the signals from the PMTs that detected scintillation photons, and position information is obtained by applying a positioning algorithm to the quantitative signals produced by the PMT array. The original gamma-ray camera is described in U.S. Pat. No. 3,011,057 issued to Hal Anger in 1961.
The CsI camera is typically used with either a single silicon-based photodiode detector or an array of silicon-based photodiode detectors, which detect scintillation events emitted from the CsI crystal. CsI crystals are used where the relatively low cost, ruggedness and spectral response of the CsI crystal are desired in favor of alternative crystal materials such as NaI.
In PET imaging, scintillation crystals with short response times are required in order to properly detect the coincidence events with high timing resolution. Typically such crystals are chosen from among materials such as NaI, BGO, LSO and BaF2. Detectors coupled to such crystals can be an array of PMTs, a single “position-sensitive” PMT (“PS-PMT”), or fast-response silicon-based photodiodes such as avalanche photodiodes.
Because the conventional Anger camera uses a thin planar sheet or disk of scintillation crystal material, it is necessary to cover the entire field of view of the crystal with light detectors such as PMTs or photodiodes. Additionally, because of the relatively small thickness dimension of the crystal (typically in the range of 0.5 to 2.5 cm) the sampling capability of such scintillation crystals is relatively low, in that a significant number of gamma photons emanating from an imaging subject will pass through the entire crystal without any interaction, and consequently cannot be detected for use is in constructing an image.
The bar detector is a specific configuration of scintillation detector that has been used in astronomical and high energy physics applications. The bar detector consists of an elongated scintillation crystal bar having a relatively small cross section. A photosensor such as a PMT is optically coupled to each end of the bar. The light from a gamma photon event within the scintillation crystal volume is detected by the two PMTs. The timing or signal information can be used to determine the location of the event in the bar. Additional bars can be placed next to each other for two dimensional detection.
An example of a proposed design for a PET detector module using a bar detector is given in Moses et al., “Design Studies for a PET Detector Module Using a PIN Photodiode to Measure Depth of Interaction,” IEEE Transactions on Nuclear Science NS-41, pp. 1441–1445 (1994), incorporated herein by reference in its entirety. According to this design, a scintillation bar is coupled at one end to a PMT, and at the other end to a photodiode, in order to measure the depth of interaction (DOI) of the scintillation event in the bar.
In past bar detector experiments for physics and astronomy, NaI (and sometimes CsI) bars of up to 100 cm were used to detect gamma photons of up to 10 MeV. Positional resolution within the bar ranged from 1.5 cm at 200 keV to 2 cm at 10 MeV, although worse resolutions were reported. An energy resolution of 9.4% and a timing resolution of 10 ns at 662 keV and a 100 cm NaI bar were reported by a physics group for a balloon borne gamma telescope. Energy resolutions from other experiments were higher for the same energy gamma photon. These studies have cited geometry, bar size, light attenuation coefficient and electronic noise as the major factors in determining the spatial and energy resolution of bar detectors. However, the performance of bar detectors as designed in the prior art is insufficient for use in medical imaging applications.
A so-called rotating slit gamma camera is also known in the art, see, e.g., U.S. Pat. No. 4,514,632 to Barrett, issued Apr. 30, 1985. The rotating is slit camera has an elongated slit provided in an opaque disk located between the imaging object and the detector, such that scintillation event detection is obtained only in one dimension along the length of the slit (i.e., only a single spatial coordinate is obtained) at a time. The disk is rotated with respect to the detector to obtain spatial position information along other directions. One advantage of the rotating slit camera is that it eliminates the requirement for the inefficient simple collimator or pinhole apertures in the conventional Anger camera, which greatly restrict the percentage of gamma photons emanating from an imaging object that ultimately reach the detector.
FIGS. 1–3 illustrate a rotating bar detector gamma camera with slat collimation as disclosed in co-pending U.S. patent application Ser. No. 10/633,935 by Ronald E. Malmin, and assigned to the same assignee herein. The rotating slat collimator detector uses planar integration as a method of image reconstruction. Only photons that are incident to the detector surface in the single direction parallel to the slats are collimated, and thus the detector generates a one-dimensional projection of a two-dimensional photon image. If such projections are acquired over a large number of different orientations, it becomes possible to reconstruct an image using the inversion of the 3-D Radon transform, as described in Chiu et al., “Three Dimensional Reconstruction from Planar Projections,” J. Opt. Soc. Amer., Vol. 70, No. 7, pp. 755–762 (1980).
Referring to FIGS. 1a and 1b, a gamma camera detector 100 is constructed of a stack of scintillation bars 101. Each scintillation bar 101 is a narrow strip made of appropriate material such as CsI, NaI, LSO, LaBr3, LaCl3, etc.
Each bar 101 is collimated by a “slat” collimator 103, which collimates gamma photons in one dimension only (i.e., along the length of the bar).
Light photons generated by absorption of gamma rays within the bars 101 are collected at the ends of each bar by a pair of photodetectors 201, 202 (as shown in FIG. 2). The light photon detectors 201, 202 may be implemented as silicon drift detectors (SDDs), small area photodiodes or photodiode arrays, position-sensitive PMTs (PS-PMTs), or other solid-state photodetectors.
As shown in FIGS. 1a–b, the detector 100 is composed of a stack of narrow bar detector strips, each having the same length L, width w, and depth d. As illustrated, the width dimension w is significantly smaller than the depth d. The bar strips are each collimated by slat collimators 103. As shown, the slat 103 length and spacing matches the length and width of the bars 101. The entire slat collimator may be placed in front of the bar detector stack with respect to an imaging object (FIG. 1a), or the individual bars 101 may be located between slats 103 (FIG. 1b).
When placed adjacent to an imaging object that is emitting gamma radiation, each collimated bar will absorb gamma photons from a plane within the object. Gamma absorptions within each narrow strip produce a number of light photons that travel along the length of the bar strip in each direction, and are collected at the ends of the bar strip by the pair of SDDs 201, 202. Because the slat collimators collimate gamma photons in only one dimension (along their length), high position resolution is required in only the dimension perpendicular to the collimated bars. Consequently, a desirable value for the width w of the bar detector strips for contemplated medical imaging applications is on the order of 3 mm.
Because the slat collimators collimate gamma photons in only one dimension, the stack of bar detectors collects a set of planar integrals at each rotational position, as opposed to the line integrals that are collected by the conventional PMT arrays of the conventional Anger gamma camera. The bar detector stack 100 is positioned at a fixed gantry angle, and collects a sufficient number of events at its initial azimuthal position. The bar detector stack 100 then is rotated azimuthally about its central normal axis 203 as shown in FIG. 2. The bar detectors may be rotated through a total rotation angle of 180 degrees in increments, such as 3–5 degree rotational increments. The bar detectors then collect additional sets of planar events at each of the rotation angle increments. As shown in FIG. 3, the process is repeated at a number of different gantry angles 401, 403, 405, and 407 with respect to an imaging object 402 such as a patient undergoing medical is imaging. The resulting sets of planar integrals can be reconstructed to form a full tomographic image of the object 402.
Use of the stack of bar detector strips 100 in a rotating slat collimator configuration exhibits several desirable characteristics. As explained above, because of the one-dimensional nature of the detection, high positional resolution is required only in the dimension perpendicular to the slat collimators, and thus a narrow width bar detector strip on the order of 3 mm may be used, with light photon collection at each end. Position information along the bar is not required. Because light produced by scintillation events in each bar is channeled within the small area of each bar, extremely high count rates are possible without pileup from spatially separated events, especially if each bar detector is provided with its own detector readout electronics (in the case of PS-PMTs, each PS-PMT would detect light from multiple bars, with a corresponding reduction in the maximum count rate).
The reduction in pileup in turn allows the use of slower scintillator materials, such as CsI, which are well-matched to compact photodetectors such as photodiodes and SDDs. Because it is not necessary to determine spatial positioning of individual events along the length of the scintillation bars for image construction, no positioning calculations are required for imaging. Simple, fast positioning algorithms utilizing light collection ratios between the pair of photodetectors 201, 202 allow sufficient spatial resolution along the length of the bar to be utilized for the purpose of performing energy correction (e.g., energy correction based on spatial position of the event) to improve system energy resolution.
Because the depth of the bar detector does not affect spatial resolution, high spatial resolution is possible at high gamma ray energies, and the septa (slat collimators) can be made thick without causing septal artifacts in the resulting images.
There remains a need in the art to improve upon the design of a rotating bar detector gamma camera with slat collimation for specific imaging modalities.